[104697-92-9]  · C26H33PSi  · Tris(o-tolyl)[(2-(trimethylsilyl)ethylidene]phosphorane  · (MW 404.65)

(cis-selective Wittig reaction for synthesis of allylsilanes;1 with modification of substituents on phosphorus and silicon atoms, b-silylphosphorus ylides give allylic alcohols diastereoselectively2)

Preparative Methods: prepared in situ from tris(o-tolyl)[2-(trimethylsilyl)ethyl]phosphonium iodide and n-Butyllithium in THF.1 Tris(o-tolyl)phosphorus methylide (prepared in situ from methyltris(o-tolyl)phosphonium bromide and n-BuLi in THF) and (Iodomethyl)trimethylsilane were heated under reflux in THF for 5 h. The mixture was poured into H2O and the phosphonium salt was extracted with CH2Cl2. Evaporation of the organic solvent gave an oily residue which was crystallized from THF-Et2O (mp 111-113 °C). To a stirred suspension of the phosphonium salt in THF (6 mL g-1) was added 1 equiv of n-BuLi under a nitrogen atmosphere at 0 °C and the mixture was stirred for 30 min at rt, resulting in a clear yellow ylide solution.

Handling, Storage, and Precautions: unstable to air and moisture. Should be handled under a dry nitrogen atmosphere.

Wittig Reaction with (Z) Selectivity.

Phosphorus ylides having silyl groups at the b-position react with aldehydes by two pathways: a Wittig reaction and a vinylation reaction that affords allylic alcohols via migration of the silyl group and elimination of phosphine. [2-(Trimethylsilyl)ethylidene]triphenylphosphorane has been used for synthesis of allylsilanes from aldehydes.3 The reaction, however, shows low (Z/E) selectivity. In addition, allylic alcohol is formed as byproduct.3b,4 With modification of the substituents on the phosphorus atom from phenyl to the bulkier o-tolyl group, the title reagent reacts with aliphatic saturated aldehydes to give allylsilanes with high (Z) selectivity and without formation of allylic alcohols (eq 1).1

Diastereoselective Alkenylation of Aldehydes.

In the dichotomous reactivity of b-silylphosphorus ylides, electron-donating groups on the phosphorus atom and electron-withdrawing groups on silicon effect the selective formation of allylic alcohols over allylsilanes.2 Tris(4-methoxyphenyl)[(2-(methyldiphenylsilyl)ethylidene]phosphorane [110598-64-6] reacts diastereoselectively with chiral aldehydes to afford allylic alcohols with suppression of the Wittig reaction. With 2-phenylpropanal the erythro (syn) product predominates2 and with a-benzyloxy aldehydes (eq 2)5 and a,b-epoxy aldehydes6 the ylide gives erythro (anti) product selectively via the Felkin-Anh model. With a-N-Boc-amino aldehydes, [2-(trimethylsilyl)ethylidene]triphenylphosphorane gives the threo (syn) product preferentially through Cram's chelation transition state.7

Diastereoselective isopropenylation2 and (E) propenylation (eq 3)8 are also possible using b-silylphosphorus ylides substituted with methyl groups at the a- and b-positions, respectively. The latter ylide was prepared in situ by addition of methyldiphenylsilyllithium to the propenylphosphonium salt.8

Enantioselective vinylation of aldehydes (up to 92% ee) was reported by using a b-silylphosphorus ylide carrying a chiral ferrocenyl group on the phosphorus atom.9

1. (a) Iio, H.; Ishii, M.; Tsukamoto, M.; Tokoroyama, T. TL 1988, 29, 5965.
2. (a) Tsukamoto, M.; Iio, H.; Tokoroyama, T. TL 1985, 26, 4471. (b) Tsukamoto, M.; Iio, H.; Tokoroyama, T. CC 1986, 880.
3. (a) Seyferth, D.; Wursthorn, K. R.; Mammarella, R. E. JOC 1977, 42, 3104. (b) Seyferth, D.; Wursthorn, K. R.; Lim, T. F. O.; Sepelak, D. J. JOM 1979, 181, 293. (c) Fleming, I.; Paterson, I. S 1979, 446.
4. (a) Hughes, L. R.; Schmid, R.; Johnson, W. S. Bioorg. Chem. 1979, 8, 513. (b) Danishefsky, S. J.; Deninno, M. P.; Phillips, G. B.; Zelle, R. E.; Lartey, P. A. T 1986, 42, 2809.
5. Iio, H.; Mizobuchi, T.; Tsukamoto, M.; Tokoroyama, T. TL 1986, 27, 6373.
6. Iio, H.; Mizobuchi, T.; Tokoroyama, T. TL 1987, 28, 2379.
7. Franciotti, M.; Mann, A.; Taddei, M. TL 1991, 32, 6783.
8. Tsukamoto, M.; Iio, H.; Tokoroyama, T. TL 1987, 28, 4561.
9. Iio, H.; Fujii, A.; Ishii, M.; Tokoroyama, T. CC 1991, 1390.

Hideo Iio

Osaka City University, Japan

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